The present invention pertains generally to apparatus for testing the combustive properties of black powder, and pertains more specifically to apparatus for doing so by measuring the flamespread in a black powder sample.
There are several possible approaches to the determination of the black powder function that also lend themselves to the idea of a single end-of-the-line qualification test. This function can be tracked in a tester by measuring any one of six parameters. First, it is possible to monitor the rate of change of the pressure in a surrounding chamber into which hot gases are emitted from the sample tube. (As is well known, the function of the sample tube, which simulates a primer tube, is to emit hot gases at predetermined rates m(n,t) at each of n orifices as a function of time t.) This measurement in effect actually sums the mass flows from all of the orifices. Second, the rate of advance of the pressure wave in a packed bed of inert propellant simulant in the surrounding chamber can be measured. This simulates the actual advance of pressure in a live cartridge. Third, the advance of pressure along the sample tube, which is a function of the flamespread rate and of the burning rate of the powder, can be measured. Fourth, the flamespread rate itself can be measured directly. Fifth, the photo-active flash intensity output from each of the n orifices can be monitored as a function of time. Sixth, the heat intensity coming from each orifice can be measured as a function of time by means of fast response heat transfer gauges, each of which measures the output of a respective orifice.
Another possible approach to qualifying black powder would be to measure the relative quickness and/or induction time in a closed bomb-type firing. This approach has the advantage of using well-known equipment and techniques. The flamespread rate can be shown theoretically to be related to powder ignitability, as measured by induction time, and to burning rate, as measured by relative quickness.
However, it is not clear that an apparatus that was really designed to measure the burning rate, and not the flamespread rate, is suitable for use in qualifying black powder for its proper function in a gun. Black powder is not the propelling charge of the gun, and a device that measures relative quickness may therefore not necessarily be measuring the correct functional property of the powder. For example, it is conceivable that a particular powder sample might have a slow burning coating and a fast burning interior. Such a sample, if subjected to a closed bomb-type firing, would show a high relative quickness, although it would have a relatively low flamespread rate. The result of the closed bomb test in such a case might therefore be misleading. For this reason, the conventional closed bomb firing does not appear to be readily adaptable for qualifying black powder.
The six types of measurements listed above, have direct physical significance to the black powder function in a gun. Each of them should therefore theoretically be suitable for the task at hand. Not all of these methods, however, appear to be equally simple, reliable and free of error. We have determined that overall, the direct determination of the flamespread rate itself is the most advantageous approach.